In modern microelectronic systems, there is a wide variety of possible applications for storage elements in which information can be stored in the form of a pattern of localized regions of varying physical properties. Of particular interest are such elements in which the storage can be done by illuminating the storage element with a pattern of light because of the relative ease of this storage technique. It is also generally desirable that the storage pattern be stable for usefully long periods of time but easily erased for subsequent storage later of new patterns in a non-destructive fashion.
One application that would be particularly useful if irradiation for a short time by a light pattern could be used is to establish a stable, but readily erased, corresponding pattern of localized conductive regions near the surface of a semiconductive element otherwise of relatively high resistivity. Such a pattern could be used to provide between a set of input and a set of output terminals on such surface an interconnection pattern of conductive regions that could be readily modified as needed.
Another example of an application that would be useful if a light pattern could be used to create stable, but easily changed, localized regions of different refractive indices near the surface of or in the bulk of a semiconductive element is a diffraction grating. A light beam either reflected from the surface or transmitted through the bulk could be diffracted by such grating in a selective manner and the diffracted beam utilized appropriately.
In particular, photorefractive materials that exhibit a persistent change in their refractive index on exposure to light are useful as potential media for the storage of both data and interconnection patterns for massively parallel computers. See for example, a paper by D. Brady and D. Psaltis that appeared in the Journal of the Optical Society of America, A9, #7, 1167 (1992). In the materials typically considered for these applications, such as barium titanate and lithium niobate, illumination causes an inhomogeneous distribution of trapped charges. Electrons diffuse out of the illuminated regions and are trapped by defects in the dark regions giving rise to a space charge and electric fields. The internal electric fields resulting from this charge distribution then modulate the refractive index through the electro-optic effect. Although this change is only about 2.times.10.sup.-4, demonstrations involving the simultaneous storage of up to 5000 images in one crystal of lithium niobate have been reported. See for example, a paper by F. J. Mok in Optic Letters 18(11) Jun. 1, 1993 p. 915.
Presently there is considerable interest in using zinc-blende semiconductors as photorefractive materials because of ther low cost. The EL2 defect center in semi-insulating GaAs has been shown to be useful as a photorefractive center in the presence of acceptor impurities which result in a distribution of EL2.sup.+ and EL2.degree. centers and in the possibility of electron exchange between them. The concentration of EL2 centers that can be practically realized is however limited to about 1.5.times.10.sup.16 per cm.sup.3. Moreover, EL2 exhibits a light-induced metostability which can interfere with this charge exchange. It also appears that EL2 centers do not lead to persistent photoconductivity.
A recognized problem in the use of some n-type doped crystalline compound semiconductive materials is that they exhibit so-called deep donor DX states or centers. The observed effects, which include severe carrier freezeout even at room temperature, have been explained in terms of a large lattice relaxation model and charge capture by the donor. In particular, it is known that in materials that exhibit these phenomena, the total energy of the donor atoms that have captured an electron and become negatively charged is lowered. In these materials, the DX state becomes the ground state of the system and a reduction of free carrier concentration by orders of magnitude from the impurity doping level is often observed at sufficiently low temperatures. As a result even heavily doped material may be rendered essentially insulating. Moreover, it has been shown that, in such materials, persistent photoconductivity (PPC) can occur when the DX states are ionized by photons of appropriate energy. As used herein, PPC means persistence for a time long enough after the illumination has been removed to be useful in the intended device application. For some device applications, persistence of a few seconds may be sufficient, for others persistence for hours may be needed. Generally, the duration of the persistence is a function of the temperature at which the material exhibiting the PPC is maintained, the colder the temperature the longer the persistence, as will be discussed more fully below. Upon photoexcitation each DX center is converted into a positively charged impurity ion, releasing two electrons into the conduction band. A barrier to recombination is formed by the structural relaxation required to return to the deep DX state. If the ambient temperature is sufficiently low, thermal excitation over the capture barrier occurs at a very slow rate and the free carrier concentration can remain high for long times resulting in PPC. The concentration of DX centers is nearly the same as that of the donor impurities and concentrations of 5.times.10.sup.18 per cm.sup.3 or higher can be achieved. In the original formulation of this model, it was proposed that a donor atom (D) forms a complex with an unknown lattice defect (X). It has subsequently been shown that the donor atom alone, through a distortion in the crystalline lattice associated with electron capture, is responsible for all of the observed phenomena. Nevertheless the "DX" terminology persists among workers in this field.
Among the most important materials from a device standpoint that exhibit these phenomena are Al.sub.x Ga.sub.1-x As where x is greater than 0.22 and preferably below 0.40, n-type GaAs.sub.x P.sub.1-x alloys, and group-II group-VI semiconductive compounds, such as Cd.sub.x Zn.sub.1-x Te, Cd.sub.x Mn.sub.1-x Te, and those involving selenium and CdS alloys. Moreover, theoretical studies suggest that n-type GaN, AlGaN and SiC, and CuCl should have similar properties, operating at even higher temperatures.
In the past these DX-PPC phenomena have been viewed as a problem to be avoided and little effort appears to have been made to utilize them beneficially for device applications.